The present invention relates generally to the treatment of materials under pressure, and more particularly, to the refining of fibrous cellulosic materials under conditions which permit the process to be carried out on a substantially continuous basis.
Cellulosic fibers must be subjected to mechanical treatment before they can be made into paper. This treatment may be applied in a number of different ways, but it generally includes a rubbing, distortion or crushing action on the fibers. The terms beating and refining are often used in the paper industry to describe the operation of mechanically treating pulp fibers. Refining usually refers to a fiber separation and fiber cutting action, whereas beating action may include these two effects, and also a fibrillating effect on the fibers. The amount and type of mechanical treatment used on the pulp contributes to the final pulp properties including burst, tensile strength, density, bulk, freeness and formation. In this regard, refining with a disk refiner is an effective treatment for enhancing the properties of paper pulp.
In disk refining, the pulp generally enters the refiner at a solids content of about 4-5%. For this purpose, the refiner housing has an inlet arranged to direct the pulp through an opening near the center of one disk, from which it passes into a central feeding chamber. Centrifugal forces at that point force the pulp outwardly between the disks where the refining action takes place before the treated pulp is discharged at the periphery of the housing. Pulp throughout is controlled by the amount of material introduced into the refiner, while the gap or space between the refiner disks determines the amount of work or energy expended on the pulp.
Because of the water present in and with the pulp which is admitted into the refiner housing, great amounts of steam are generated as energy is added during the refining operation. The build up of steam and pressure in the refiner housing is partially relieved when some of the steam passes out of the refiner space with the treated pulp. Sometimes the steam flowing outward together with the fibrous material assumes a very high speed, however, fiber bundles or discontinuities produced during refining may plug the refiner and obstruct the flow through the refiner outlet, thus producing an abnormal amount of pressure in the refiner housing. Under these conditions, safeguards must be provided to alleviate the high pressures. In many cases, the refiner may need to be shut down for correcting the problem that caused the high pressures. However, often the refiner can be continued in service by temporarily relieving the excess pressure.
An example of a means for relieving steam pressure in a refiner is disclosed in U.S. Pat. No. 4,221,631. In this patent, passageways are provided in each of the refining surfaces of the refining disks so that the steam generated during refining may be withdrawn without also withdrawing substantial quantities of unrefined fibrous material. Meanwhile, in U.S. Pat. No. 4,236,959, the discharge of refined material is controlled by an exhaust valve capable of adjusting the size of the outlet in response to signals from a regulator connected to a pressure sensing instrument located in the refiner housing. Also, in U.S. Pat. No. 2,293,670, there is disclosed a refiner apparatus which includes a rupturable diaphragm in the conduit for venting the combustion products of fires that might occur during refining. In this connection, it is also known to install a pressure relief means in the form of a rupturable disk in association with the outlet of a refining device which is designed to rupture and relieve pressure in the refiner when abnormally high pressures are reached. However, when only a single rupture disk is included in the discharge conduit of a refiner, the refiner must be shut down in order to replace the rupture disk when a disruption occurs. This is a decided disadvantage when it is desired to keep the refiner operating on a substantially continuous basis.